Today, as a power supply device installed in a vehicle, a power supply device that feeds power to a load via two current paths (e.g., see JP 2012-29465A) has been proposed.
In the power supply device described in JP 2012-29465A, one of the two current paths is provided with a DC/DC converter. The DC/DC converter transforms an applied voltage, and feeds the transformed voltage to a load. Consequently, the load is supplied with power. The other current path is not provided with a DC/DC converter, and power is directly fed to the load. In the power supply device of JP 2012-29465A, the current path used to feed power to the load is switched to one of the above-described two current paths.
FIG. 1 is a block diagram showing the configuration of a relevant portion of a conventional power supply device 8. In the conventional power supply device 8, one end of a generator 80 is connected to one end of each of a switch 81 and a DC/DC converter 82, and the other ends of the switch 81 and the DC/DC converter 82 are connected to one end of a resistor R8. The other end of the resistor R8 is connected to one end of a load 83. The other ends of the generator 80 and the load 83 are grounded. One end and the other end of the resistor R8 are connected to a positive terminal and a negative terminal, respectively, of a differential amplifier 84, and the output terminal of the differential amplifier 84 is connected to an A/D (Analog/Digital) converting portion 85. The A/D converting portion 85 is further connected to a control portion 86.
The control portion 86 turns the switch 81 on/off and activates/deactivates the DC/DC converter 82. When the control portion 86 turns the switch 81 off to activate the DC/DC converter 82, the DC/DC converter 82 lowers a DC output voltage output by the generator 80, and supplies the lowered voltage to the load 83 via the resistor R8. When the control portion 86 turns the switch 81 on to deactivate the DC/DC converter 82, the generator 80 supplies the output voltage to the load 83 via the switch 81 and the resistor R8.
The differential amplifier 84 amplifies the voltage value across the resistor R8, and outputs the amplified analog voltage value to the A/D converting portion 85. The output voltage value of the differential amplifier 84 increases with an increase in the value of the current flowing through the resistor R8. The A/D converting portion 85 converts the analog voltage value into a digital voltage value, and outputs the converted digital voltage value to the control portion 86. Since the voltage value across the resistor R8 is proportional to the value of the current flowing through the resistor R8, the resistor R8, the differential amplifier 84, and the A/D converting portion 85 function as a current detecting circuit.
Normally, the control portion 86 turns the switch 81 off to activate the DC/DC converter 82. The value of the current that can be passed via the DC/DC converter 82 is limited to a predetermined value. Therefore, a first current value less than the predetermined value and a second current value greater than or equal to the first current value are set. If the switch 81 is off and the DC/DC converter 82 is activated, the control portion 86 deactivates the DC/DC converter 82 to turn the switch 81 on when the value of the current flowing through the resistor R8 become greater than or equal to the first current value and the voltage value output by the A/D converting portion 85 becomes greater than or equal to the first voltage value. Consequently, the current path of the current flowing from the generator 80 to the load 83 is switched from a first path through which the current flows via the DC/DC converter 82 to a second path through which the current flows via the switch 81. In this manner, the control portion 86 switches the current path, thus making it possible to continuously supply a current to the load 83 even if the load 83 needs the supply of a current having a value greater than or equal to the predetermined value.
If the switch 81 is on and the DC/DC converter 82 is deactivated, the control portion 86 turns the switch 81 off to activate the DC/DC converter 82 when the value of the current flowing through the second path becomes less than the second current value and the voltage output by the A/D converting portion 85 becomes less than second voltage value. Consequently, the current path of the current flowing from the generator 80 to the load 83 is switched from the second path to the first path. The voltage lowered by the DC/DC converter 82 is supplied to the load 83 again. The current detecting circuit and the control portion 86 described above function as a switching device that switches the current path through which a current flows, to the first path or the second path.
The A/D converting portion 85 performs processing such as rounding or truncation to quantize the voltage value output by the differential amplifier 84 to a voltage value that is closest or second closest to the voltage value output by the differential amplifier 84, from among the voltage values obtained by equally dividing a reference voltage value by a predetermined number. In the conventional power supply device 8 in which the current path is switched in the above-described manner, a current having a value less than the first current value flows through the first path, and a current having a value greater than or equal to the second current value flows through the second path. Since the second path is provided to pass a current having a value greater than or equal to the first current value, a current having a value greater than or equal to the first current value flows through the second path.
For example, when a current of zero to 100 A flows through the first path and a current of 90 to 200 A flows through the second path in the conventional power supply device 8, a current of zero to 200 A flows through the resistor R8. When the voltage value output by the differential amplifier 84 if a current of 200 A flows through the resistor R8 is 10 V, the voltage value output by the differential amplifier 84 varies in the range of zero to 10 V. Accordingly, the reference voltage value of the A/D converting portion 85 is set to be greater than or equal to 10 V. If the reference voltage value is 10 V and the A/D converting portion 85 converts the voltage value into a 10-bit digital value, the A/D converting portion 85 quantizes the voltage value output by the differential amplifier 84 to a voltage value that is closest or second closest to the voltage value output by the differential amplifier 84, from among voltage values obtained by equally dividing 10 V (the reference voltage value) by 1023 (the predetermined number). At this time, the interval between markings is about 9.78 mV.
The value of the current flowing through the first path varies in the range of zero to 100 A. Let us now consider a configuration in which only the value of the current flowing through the first path is detected. When the voltage value output by the differential amplifier 84 if a current of 100 A flows through the resistor R8 is 5 V, the reference voltage value of the A/D converting portion 85 can be set to 5 V. At this time, the A/D converting portion 85 quantizes the voltage value output by the differential amplifier 84 to a voltage value that is closest or second closest to the voltage value by the differential amplifier 84, from along voltage values obtained by equally dividing 5 V (the reference voltage value) by 1023 (the predetermined number). At this time, the interval between markings is about 4.89 mV. When the reference voltage value is 5 V, the interval between markings is small. Accordingly, the current detecting circuit composed of the resistor R8, the differential amplifier 84, and the A/D converting portion 85 can highly accurately detect the value of the current flowing through the first path.
However, in the current detecting circuit installed in the conventional power supply device 8, the reference voltage value of the A/D converting portion 85 is set in accordance with the maximum value of the current flowing through the second path, thus resulting in the problem that the value of the current flowing through the first path cannot be detected highly accurately. In this case, the current path may not be switched appropriately.
To solve this problem, it is conceivable to adopt a configuration in which the number of each of the resistor, the differential amplifier, and the A/D converting portion is increased to two, and the values of the currents flowing through the first path and the second path are separately detected. However, this configuration requires a large number of components, and therefore has the problem of an increased manufacturing cost.
The present invention has been made in view of such circumstances, and it is an object of the invention to provide an inexpensive current detecting circuit capable of outputting a voltage value accurately indicating the value of a current flowing through each of the two current paths, as well as a current detecting device and a switching device each including the current detecting circuit.